1. Field of the Invention
This invention relates to a method of forming a microscopic pattern, and more particularly to a method which can form a microscopic pattern at high precision far more easily than in a prior art by the use of the lift-off process.
2. Description of the Prior Art
With remarkable enhancements in the densities of integration of various semiconductor integrated circuits in recent years, a technique capable of forming a microscopic pattern of very small line width and interval at high precision has increasingly been required.
Heretofore, various patterns of various semiconductor integrated circuits have generally been formed by a method called photolithography.
As to the photolithography, various processes are performed according to objects to-be-etched or purposes. Among them, the so-called lift-off process has the merit of being capable of forming the microscopic pattern at high precision. Therefore, it is extensively used for the formation of the interconnections of various semiconductor integrated circuits, etc.
An example of the steps of the lift-off process is illustrated in FIGS. 1(a)-1(e). First, as shown in FIG. 1(a), on the surface of a silicon substrate, an insulating film or the like 1 on which a microscopic pattern is to be formed, first and second photoresist films 2 and 3 whose solubilities for a developer are different from each other are deposited into stacked layers (the first photoresist film 2 can be replaced with a film of a material other than a photoresist).
The second photoresist film 3 is selectively exposed to light through a mask, and is developed to remove desired parts of the second photoresist film 3 as shown in FIG. 1(b). Since the developer used in this case does not dissolve the first photoresist film 2, only the desired parts of the second photoresist film are selectively removed.
Subsequently, using a developer which can develop only the first photoresist, the exposed parts of the first photoresist film 2 are removed. In this case, the developing is conducted somewhat excessively to cause the side etching, so that overhangs owing to the second photoresist film 3 are formed as shown in FIG. 1(c).
A material to form the microscopic pattern, for example, aluminum is deposited on the whole surface by the vacuum evaporation or the like.
Since, however, the overhangs of the second photoresist film 3 are formed, the evaporated aluminum does not reach the rear sides of the overhangs with the result that the deposited aluminum film 4 is separately deposited on the substrate 1 and the second photoresist film 3 without becoming a continous film as seen from FIG. 1(d).
Subsequently, the first photoresist film 2 is removed along with the second photoresist film 3 and the aluminum film 4 deposited thereon. Then, only the aluminum film 4 deposited directly on the substrate 1 remains, and the pattern of the aluminum is formed.
Although the lift-off process has the advantage of being capable of forming the microscopic pattern precisely, it is very complicated in practical use because it must stack and deposit the two sorts of photoresist films and carry out the developings twice as described above. A simpler method has been eagerly desired.